That said, one of my VC friends defines an Entrepreneur as someone who can take a stupid idea and make it successful. Sort of an homage to the whole idea that they can see something you cannot.
These days when I'm confronted by this sort of proposal I ask the person making the proposal to describe the world where their idea has succeeded. What is different about that world from today, and what is the same. What do people do in that world that they do differently now or can't do at all. And perhaps more importantly what do they do today which they no longer have to do.
If you can paint that picture in enough detail you can figure out who is going to be working against this person and who is going to be cheering this person on, you might get a sense of how much work is between here and there and the kinds of people needed to do that work. And if you know this person well you might be able to guess whether or not they are up for that, or perhaps more importantly, committed enough.